The present invention relates to a two-line phase-matched Raman amplifier.
There is a need for sources of high power, high beam quality, coherent radiation throughout the ultraviolet, visible, and infrared spectra for applications in strategic and tactical weaponry, isotope separation, communications, spectroscopy, photochemistry, plasma diagnostics, ets. In cases where high power lasers cannot provide the desired wavelength and beam quality for a particular application, stimulated Raman scattering of the laser radiation has often been used to change the wavelength and radiation to the needed values. A great variety of Raman devices for accomplishing this have been described in review papers, e.g., "Stimulated Raman scattering of laser radiation with a wide angular spectrum," by Yu. E. D'yakov and S. Yu. Nitikin in the Soviet Journal of Quantum Electronics, v.17, p. 1227-1247 (1987). "High Power tunable IR Raman lasers," by A. Z. Grasiuk and I. G. Zubarev, in Applied Physics V.17, pp.211-232 (1978), and "Raman lasers," by A. Z. Grasyuk, in the Soviet Journal of Quantum Electronics, v.4, p.269-282 (1974). Particular problems may occur with these previous approaches, however, when the laser has two strong spectral lines, e.g., a high temperature XeF excimer laser. Some of these problems have been described in "Two-line coupling beam quality effects in stimulated Raman scattering," by R. B. Holmes and A. Flusberg, in the Proceedings of the Society of Photooptical Instrumentation Engineers v. 642, pp.143-148 (1986).
In a Raman amplifier with a two-line pump laser, dispersion in the Raman medium may decouple the Raman gains from each other. The small signal gain of each line is then reduced by the fraction of the total pump power in each line. If these gains are very different from each other, then the overall conversion efficiency to the Stokes-shifted output may be limited by the difficulty in optimizing the amplifier cell length for both lines simultaneously. Incomplete coupling can lead to wavefront aberrations through interaction with self-focusing induced by a transient refractive index in the medium.
United States patents of interest include U.S. Pat. No. 4,165,469 to Ammann, which discloses a Raman crystal which produces second harmonics and sum frequencies of the pump and Stokes lines. By rotating the crystal to change the angle it forms with the pump beam it is possible in the patented construction to phase match the second harmonics and sum frequencies. Eckbreth, in U.S. Pat. No. 4,277,760, generates coherent anti-Stokes Raman radiation where three input beams are focused to produce three-wave mixing. The angular separation between the beams is controlled to phase match the beams. Two of the beams in the Eckbreth Patent are pump beams and the third is a Stokes beam. In U.S. Pat. No. 4,361,770 Rabinowitz et al discuss the four wave mixing that occurs in a Raman device involving two pump beams and two Stokes beams. In U.S. Pat. No. 3,881,115, Hodgson et al describe a double quantum laser employing a combination of Raman emission and four-wave parametric conversion. In U.S. Pat. No. 4,498,051, Hunter et al discuss pumping a krypton fluoride lasing media.